A multilayer ceramic capacitor which is one example of multilayer ceramic electronic parts is illustrated in FIG. 1, wherein an external electrode (4) formed on a surface where an internal electrode(s) of a ceramic composite body of the multilayer ceramic capacitor (1) is led out is generally formed by using a fired-type conductive paste or a thermosetting conductive paste according to a method as described below, wherein the ceramic composite body comprises a ceramic dielectric(s) (2) and the internal electrode(s) (3), as alternately layered.
The first method is the one for forming an external electrode (4) by applying a fired-type conductive paste to a surface where an internal electrode(s) (3) of a multilayer ceramic composite body is led out, wherein the fired-type conductive paste is prepared by blending conductive particles such as silver or copper powder and glass frit into a vehicle; and drying the same; and thereafter firing the same in an atmosphere of inert gas such as nitrogen gas, at an elevated temperature between 500 to 900° C.
The second method is the one for forming an external electrode (4) by applying a thermosetting conductive paste to a surface where an internal electrode(s) (3) of a multilayer ceramic composite body is led out, wherein the thermosetting conductive paste is prepared by blending conductive particles such as Ag powder into a thermosetting resin; and thereafter thermosetting the same at a low temperature between 150 to 250° C. [See, for example, JPA-(Japanese Patent KOKAI (Laid-Open) No.) Hei 6(1994)-267784].
The third method is the one for forming an external electrode (4) by applying a thermosetting conductive paste to a surface where an internal-electrode(s) (3) of a multilayer ceramic composite body is led out, wherein the thermosetting conductive paste is prepared by blending a pyrolytic organometallic material(s) such as silver acetate and conductive particles such as Ag powder into a thermosetting resin; and thereafter thermosetting the same at a temperature of 350° C. (See, for example, JPA-2000-182883).
In any method, in order to enhance the bonding strength when the capacitor element obtained is mounted on a substrate or the like with solder, if necessary, a plating-treated layer (5) may be formed upon the surface of the electrode layer. For example, the surface of an external electrode may be electroplated with nickel in a Watt bath or the like, and thereafter further may be electroplated with solder and/or tin.
However, a capacitor having an external electrode(s) obtained by the first method above has, for example, the problem of poor plating-adhesion due to the floating of glass, because of the diffusion of the glass-frit component in the conductive paste into the inside of a capacitor element when the capacitor is fired at an elevated temperature, and the problem of the occurrence of cracks when the capacitor is mounted with solder on a substrate. Furthermore, it has problems concerning the reliability of capacitor performance. For example, due to the infiltration of a plating solution into a sintered body in the plating-treatment, the electric capacitance is lower than a designed value, and/or the insulation resistance is deteriorated.
On the other hand, a capacitor having an external electrode(s) obtained by the second method above, wherein the thermosetting conductive paste is used and thermally set at the low temperature, can solve the problems at the time of its mounting on the substrate or of its plating-treatment as described above. However, because of its low curing temperature, the metal-to-metal solid-phase diffusion of the conductive particles (such as silver powder) in the conductive paste into the internal electrode(s) can not progress, and thus the junction of the internal electrode(s) with the external electrode(s) is poor, thereby its designed electrical properties such as electric capacity can not be obtained, and thus the resultant capacitor is poor in the reliabilities.
Furthermore, a capacitor having an external electrode(s) formed by the third method, wherein the thermosetting conductive paste containing the pyrolytic organometallic material(s) is used, has such problems as the insulation deterioration for its moisture-resistible duration of life and the pot life of the paste is shortened, due to silver acetate and amine added to the paste.
It is an object of the present invention to solve the above-mentioned problems which the prior art has had in the formation of an external electrode and the sequential plating-treatment. That is, it is the object of the present invention to solve the problems concerning bonding-property between an internal electrode(s) and the external electrode, which the above-mentioned thermosetting conductive paste has had, and to provide a multilayer ceramic electronic part having high reliabilities which is suitable for its mounting on a substrate and for its plating-treatment.